Transmission cables are used to convey electronic signals from a source device to a destination device, e.g., a display terminal. The cable may not accurately convey the signals because of losses which accumulate along the cable path. These losses are primarily due to the physical characteristics of the transmission cable and sometimes due to imperfections in the cable construction. The imperfections may not necessarily be due to manufacturing deficiencies, but may also be due to the fact that a cable is a physical device and most physical devices exhibit some losses when a signal is conveyed through them. Thus, longer length transmission cables tend to exhibit more loss (also known as “cable insertion” loss) than shorter length cables. A length limit exists for each transmission cable medium after which a video signal may no longer be discernable.
Video may be transmitted either in digital or analog format. For digital video transmission such as computer video, cable insertion loss is generally not an issue because the digital signal can be recovered so long as discernable digital pulses are detected by a receiver. A pulse-detection method and apparatus is described, for example, in commonly owned U.S. patent application Ser. No. 11/897,547 entitled “Method and Apparatus for Improving the Quality of a Transmitted Video Signal,” which was filed with the U.S. Patent and Trademark Office on Aug. 29, 2007 and the specification of which is incorporated herein in its entirety by reference. For analog signals such as NTSC (National Television Standards Committee) video signals, the signal comprises varying voltages with the voltages being affected by wire length, connectors, heat, cold, materials, manufacturing processes, and/or other conditions.
Cable insertion loss varies with the type of transmission medium. For instance, coaxial cables are known to exhibit less insertion loss than twisted pair cables, thus coaxial cables are the medium of choice for video transmission. Also, because of its superior performance over twisted pair cables, coaxial cables are typically used for transmission of high resolution (i.e., broadband) video signals. However, coaxial cables are more expensive and difficult to install when compared to twisted pair cables.
Historically, the significant differences between coaxial and twisted pair cables limited twisted pair cables to transmission of low-resolution video (i.e., less than 10 MHz) signals. However, twisted pair cables have a distinct advantage over coaxial cables, namely, the cost/performance ratio. Dollar-for-dollar, twisted pair cables are significantly cheaper to purchase and/or install than coaxial or fiber optic cable. A standard twisted pair cable contains four pairs of conductors in a single cable so that the actual cost per pair is one-quarter of the per-foot price.
Analog video specifications such as C-Video, S-Video, or YUV (or YIQ) may be available in various color models. A color model (also known as “color space”) defines colors in some standard, generally accepted way. For example, the RGB color model includes R for the red component; G for the green component; and B for the blue component.
Data-grade twisted pair cable comes in two types: unshielded twisted pairs commonly called UTP, and STP (shielded twisted pairs). By far, most of the domestic data installations tend to employ UTP cables.
In the mid 1980's, twisted pair technology began to emerge which could transmit 2 Mbps (Megabits per second), then 4 Mbps (the original IBM data rate), and then 10 Mbps. As data rates increased, it became apparent that some means of assessing twisted pair cable performance was needed. It was at that time that a system of “Levels” was proposed. The TIA (Telecommunications Industry Association)/EIA (Electronics Industries Alliance), two groups that set standards for the communications industry, adopted the proposal and separated the data rates and other parameters into “Categories”, such as Category (CAT) 3, 4, 5, and 6. Each higher numbered category has more stringent requirements with higher data rates and higher performance than the previous category.
The specifications for each category are given in TIA/EIA-568-B. TIA/EIA-568-B is a set of three telecommunications standards published by the TIA. The standards address commercial building cabling for telecommunication products and services. The three standards are formally titled ANSI/TIA/EIA-568-B.1-2001, -B.2-2001, and -B.3-2001. The TIA/EIA-568-B standards were first published in 2001 and supersede the TIA/EIA-568-A standards set, which is now obsolete. For example, the TIA/EIA-568-B.1-2001 defines the pin/pair assignments for eight-conductor 100-ohm balanced twisted pair cabling. These assignments are named T568A and T568B.
With regard to appearance, present-day twisted pair cables look identical to POTS (Plain Old Telephone Service) cable. The cables use the same color code, come in many of the same pair counts and use the same gauge conductors. However, the specifications they are made to, the materials used to make them, and the requirements to connect them, become more and more critical as data rates increase.
The 4th pair of a CAT 5, 6, or 7 wire bundle may be used to convey power and digital communications between a transmitter to a receiver, and digital communications from the receiver to the transmitter. The digital communications have identification bits, which identify the transmitting device. A number of transmitters may transmit into the same communication link. Collision may be detected by CRC (Cyclic Redundancy Check) error checks.
High-resolution analog video such as RGB requires that each color component be transmitted separately to a destination device. For such transmission, a coaxial cable setup will require three separate coaxial cables to carry each color component and another cable to carry audio data. In contrast, a twisted pair setup only requires one twisted pair cable for all the video components, and a spare pair of conductors for audio and other communication needs. For instance, each of the three color components of the RGB format video may use one out of the four twisted pair conductors in the cable bundle, and the last (i.e. fourth) twisted pair may be used for power and/or digital communication needs. Thus, twisted pair bundles have a clear advantage over coaxial cables in terms of installation and costs.
Various methods for compensation of signal transmission errors have been used. Specifically, methods and apparatuses to compensate for phase, DC error, AC loss, and skew error are described, for example, in commonly owned U.S. patent application Ser. No. 11/309,120, filed Jun. 23, 2006, and entitled “Method and Apparatus for Automatic Compensation of Skew in Video Transmitted over Multiple Conductors,” U.S. patent application Ser. No. 11/309,123, filed Jun. 23, 2006, and entitled “Method and Apparatus for Automatic Reduction of Noise in Video Transmitted over Conductors,” U.S. patent application Ser. No. 11/309,558, filed Aug. 22, 2006, and entitled “Method and Apparatus for DC Restoration Using Feedback,” U.S. patent application Ser. No. 11/557,938, filed Nov. 7, 2006, and entitled “Method and Apparatus for Video Transmission over Long Distances Using Twisted Pair Cables,” and U.S. patent application Ser. No. 11/309,122, filed Jun. 23, 2006, and entitled “Method and Apparatus for Automatic Compensation of Video Signal Losses from Transmission over Conductors,” the specifications of which are incorporated herein in their entirety by reference.
The need exists for a video format identification system which can automatically identify the various video signal formats and switch the transmitted video signal to the proper output connector. Such system should preferably be capable of simultaneously transmitting and recovering video, audio, communication and power signals over at twisted pair cable bundle.